freestylephoto.biz sells Kodak d76 to make a gallon for $6.09 and fixer powder (makes a gallon) for $5.79

Adox Adonal (aka agfa Rodinal) 500ml for $14 is still the cheapest per use as it can be diluted to 1:50 or beyond._________________"In order to invent, you need a good imagination and a lot of junk" Thomas Edison

The cost of the chemistry is very cheap especially if you use stock XTOL and replenish with unused XTOL [70ml for every four sheets of 4"x5" film]. Your real costs are for the film, because the tank gets amortized while the chemistry and film do not.